1) Field of the Invention
The present invention relates to an electrode for discharge surface treatment for generating a pulsed electric discharge between the discharge surface-treating electrode and a workpiece in a treatment liquid or in the air using, as the discharge surface-treating electrode, a green compact obtained by compression-molding metal, metal compound, or ceramic powder or a green compact obtained by heating the green compact, and for forming a coat, which consists of an electrode material or a substance obtained by provoking a reaction of the electrode material by a discharge energy, on a surface of the workpiece using a discharge energy of the pulsed electric discharge, and a method of evaluating the electrode for discharge surface treatment. The present invention also relates to a discharge-surface-treating method using the electrode for discharge surface treatment.
2) Description of the Related Art
Recently, there is an increasing demand for a coat that has an abrasion resistance and a lubricity in a high temperature environment such as for use for a turbine blade of an aircraft gas turbine engine. FIG. 1 is a schematic diagram of a structure of the turbine blade of the aircraft gas turbine engine. As shown in the drawing, a plurality of turbine blades 1000 are brought into contact with and fixed to one another, and are constituted to be turned around a shaft (not shown). Contact portions P of the turbine blades 1000 are badly worn and stricken when the turbine blades 1000 are turned in a high temperature environment.
In such a high temperature environment (700° C. or higher) in which the turbine blades 1000 are used, a coat having an abrasion resistance and a lubricity in an ordinary temperature environment is oxidized in the high temperature environment. Therefore, the coat is hardly effective. Due to this, a coat (thick film) consisting of an alloy material that contains a metal (e.g., Cr (chromium) or Mo (molybdenum)) for generating an oxide exhibiting the lubricity in the high temperature environment is formed on each turbine blade 1000 or the like. The coat of this type is formed by a welding method, a thermal spraying method, or the like.
An operation based on each of the welding method, the thermal spraying method, and the like is manually performed and requires skill. It is, therefore, disadvantageously difficult to execute the operation as a line operation, and an operation cost is disadvantageously increased. Further, the welding method, in particular, involves a step of intensively applying heat into the workpiece (hereinafter, “work”). For this reason, the welding method has disadvantages in that a welding crack and a deformation tend to occur and yield is low if a treatment target is a thin material or a brittle material such as a single-crystal alloy or a directionally controlled alloy such as a directionally solidified alloy.
A method for forming a coat on a surface of a work using a pulsed electric discharge (hereinafter, “discharge surface treatment”) is disclosed in International Publication No. 99/58744 pamphlet and the like. In this discharge surface treatment, the coat is formed by generating an ark discharge between an electrode, which consists of a green compact obtained by compression-molding powder to be generally as hard as a chalk, and a work, and by resolidifying an electrode material molten by this arc discharge on the surface of the work. This discharge surface treatment is drawing attention as a technique for enabling a line operation in place of the welding method, the thermal spraying method, or the like. In the conventional discharge surface treatment, a coat consisting of a hard material such as TiC (titanium carbide), exhibiting an abrasion resistance at the ordinary temperature is formed.
Recently, a demand for forming not only the hard ceramic coat having the abrasion resistance at the ordinary temperature but also a thick film of 100 micrometers or more using the discharge surface treatment that can be executed as a line operation without the need of a manual and skilful operation is growing.
The method disclosed in the International Publication No. 99/58744 pamphlet is, however, mainly targeted to form the thin film that exhibits the abrasion resistance at the ordinary temperature. With this method, therefore, the coat that exhibits an abrasion resistance or a lubricity in the high temperature environment cannot be formed. Besides, in the formation of the thick film by the discharge surface treatment, it is known that supply of a material from an electrode side, welding of the supplied material on the work surface, and how the supplied material is bound with the work material are the most influential factors for the coat performances. However, the International Publication No. 99/58744 fails to clarify a supply amount of the electrode material necessary to form the thick film, electrode conditions, and working conditions.
The supply amount of the electrode during the discharge surface treatment is disclosed in a conventional technique, although the technique is a thin film forming technique (see, for example, Akihiro Goto et al., Development of Electrical Discharge Coating Method, Proc. International Symposium for Electro-machining (ISEM 13), 2001). According to the thin film forming technique, the thin film is formed by the discharge surface treatment by moving an electrode material onto a work by a single discharge and by increasing parts covered with the electrode material or a material modified from the electrode material on the work by a plurality of discharges.